The present invention relates to current output circuits capable of being used as semiconductor laser drivers and the like which are required to output a large current and operate at high speed.
A current output circuit with switch function, which includes an output current generating transistor and a switching transistor, conventionally has a problem in that potential at the drain or source of the output current generating transistor greatly varies when switching is performed.
Such great variations in the drain or source potential cause the gate potential of the output current generating transistor to be varied, if a parasitic capacitance exists between the drain and gate or the source and gate of the output current generating transistor. This produces a problem in that the value of the current output from the current output circuit varies from an intended value.
Japanese Laid-Open Publication No. 2004-271646 discloses means for suppressing such variations in output current value. Specifically, the following three methods (1) to (3) are used to prevent an output current value from deviating from a desired value: (1) the size of a transistor in a bias voltage generating circuit for generating a bias voltage, which is supplied to the gate of the output current generating transistor, is increased; (2) a switching transistor, which operates in a complementary manner, is additionally provided so as to make a current continuously pass through the output current generating transistor; and (3) the transconductance of the output current generating transistor is lowered so that the sensitivity of the output current value to variations in the gate potential is reduced.